The present invention relates generally to the field of electrical utility meters, and particularly, to electrical utility meters that incorporate digital electronics.
Electrical utility meters, or watt-hour meters, measure electrical energy consumed by a facility. Electrical utility service providers, or simply utilities, employ watt-hour meters to gather energy consumption data for customer billing and other purposes. A common form of watt-hour meter is an electronic watt-hour meter. An electronic watt-hour meter measures electrical energy consumption by sampling scaled-down versions of the voltage and current waveforms on the power lines of a facility and then performing energy consumption calculations using the sampled waveforms.
One problem encountered by electronic watt-hour meters is related to the tolerances of the meter""s voltage and current sensor circuitry. The voltage and current sensor circuitry typically consists of a set of current sensors and voltage sensors that obtain signals representative of the voltage and current waveforms on the power lines, and provide scaled-down versions of those waveforms to the digital sampling circuitry. A common type of voltage sensor is a voltage divider circuit. The input to the voltage divider is connected across the power lines and the divided output of the voltage divider is connected to the digital sampling circuitry. In this manner, the digital sampling circuitry receives a suitably scaled down version of the voltage waveform. A common type of current sensor is a current transformer that comprises a toroid having a donut-shaped core. The load current, or in other words, the current consumed by the customer""s facility, is generally routed through the center of the toroid, which causes a scaled-down version of the load current to appear on the current transformer winding.
Both voltage dividers and current transformers, as well as other voltage and current sensor circuitry, are subject to substantial performance variation from unit to unit. For example, while one current transformer may produce an output rms current of 2.2 milliamperes for a load current of 10 amperes, another current transformer may produce an output rms current of 2.5 milliamperes for the same load current. Such performance variation can undesirably introduce large error in the meter""s energy consumption measurement. In particular, performance variation of the sensor devices causes both magnitude and phase angle errors in the resulting sampled waveforms, both of which contribute to error in the energy consumption measurement.
In order to improve measurement accuracy, electronic watt-hour meters are typically calibrated during manufacture to compensate for, among other things, the performance variation of the sensor devices. The calibration process produces consistent and accurate watt-hour meters. Calibration for magnitude variation of the sensor devices is a relatively straightforward and may be accomplished in a number of ways. For example, each sample of the sampled voltage waveform may be multiplied by a calibration constant, which effectively adjusts the measured magnitude.
Phase angle error in voltage and current magnitude signals is often referred to as power factor error, because the phase angle of primary interest is the phase angle between the voltage measurement signal and the current measurement signal. When that phase angle is inaccurate, the power factor, or in other words, the cosine of the angle between the measured voltage and current, is inaccurate. Inaccurate power factor substantially reduces the accuracy of the meter.
Accordingly, it is important that meters be calibrated to account for power factor errors introduced by the meter sensor devices. Calibration for phase error in the sensor devices have been accomplished using a variety of techniques. Such various methods include the selective introduction of impedance devices into the analog measurement signal stream. The impedance devices are typically connected to the output of the current sensor circuitry. So connected, the impedance devices introduce a calibrating phase delay into the analog current measurement signal produced by the current sensor circuitry. One drawback to the use of impedance devices is that the incorporation of the required amount of impedance devices to provide an acceptable range of compensation increases the packaging cost of the meter significantly.
U.S. Pat. No. 5,017,860 to Germer et al. describes another type of power factor correction circuit that addresses the packaging cost issue by implementing the correction circuit in the digital portion of the meter. The Germer et al. device corrects power factor by altering the sampling time of either the voltage or current signals to introduce a desired phase delay. In particular, the A/D converter of the Germer et al. device samples the analog current signal at a rate of 34 KHz. The time period of each sample is then subdivided into twelve equal sub-intervals. The phase delay is accomplished by digitizing or sampling the analog waveform at select one of the twelve subintervals. Thus, if a relatively large phase delay is required, each sample may represent the digitized analog waveform from the first or second sub-interval of the sampling period. If, however, no phase delay is required, then each sample may represent the digitized analog waveform from the twelfth sub-interval in the sampling period.
The power factor adjustment in Germer et al. has several drawbacks that limit its usefulness. First, it relies on old and limited A/D conversion technology. Specifically, the Germer et al. device employs a successive approximation A/D converter, which has performance limitations. Specifically, the cost of successive approximation A/D converters for a particular sampling rate and resolution exceeds the cost achieving similar results using newer A/D conversion technology, such as sigma delta A/D conversion technology. Accordingly, the Germer et al. power factor adjustment method is tied to outdated and cost ineffective technology.
Second, the Germer et al. device performs only a single adjustment for a polyphase meter. In other words, if the meter is connected to a three phase power system, all three phases receive the same power factor adjustment. Accordingly, the Germer et al. power factor adjustment does not adequately address per phase power factor errors, which commonly exist when a separate current sensor is used for each phase.
Another drawback to the above calibration techniques arises from the fact that the calibration is often line frequency specific. Because revenue meters operate on power lines, which generally have a constant frequency, the phase angle calibration amount is typically selected to calibrate the meter for proper performance at a single line frequency. In the United States, for example, meters are calibrated to provide accurate readings at 60 Hz. Other countries, however, utilize other line frequencies, such as 50 Hz. Because power factor error can vary with frequency, a meter that has been calibrated at 60 Hz may have degraded accuracy at 50 Hz.
This drawback therefore requires that meters be separately calibrated based on the geographical area in which they are to be used. If a meter is intended for use in a country that employs a 60 Hz line frequency, then it must be calibrated specifically for that frequency. Likewise, meters intended for use in countries employing a 50 Hz line frequency must be separately calibrated. The requirement of separately calibrated meters for different applications adds complexity in manufacturing, shipping and inventory control.
Accordingly, there exist a need for a power factor adjustment method that not only avoids the problems associated with introducing impedance devices into the analog signal stream, but also is not limited to use with outdated A/D conversion technology. There is a further need for such a power factor adjustment method that separately performs an adjustment for each phase of a polyphase meter. In addition, there is a need for a power factor adjustment technique that is compatible with different line frequencies.
The present invention fulfills the above stated needs, as well as others, by providing a meter having a power factor compensation technique that inserts a delay into the digitized current or voltage sample stream. That power factor compensation technique of the present invention is compatible with a wide variety of A/D conversion technologies. Moreover, the power factor compensation technique is operable to provide or separate amount of calibration adjustment for each phase of the meter.
An exemplary embodiment of the present invention includes an electronic watt-hour meter comprising a voltage sensor, a current sensor, a conversion circuit, and a processing circuit: The voltage sensor generates a voltage measurement signal responsive to a voltage provided to a load. Similarly, the current sensor generates a current measurement signal responsive to a current provided to a load. The conversion circuit further comprises: a first converter connected to the voltage sensor for generating sampled voltage data stream based on said voltage measurement signal; a second converter connected to the current sensor for generating a sampled current data stream based on said current measurement signal, and a phase correction circuit. The phase correction circuit is connected to one of the first and second converters and inserts a delay into one of the sampled voltage data stream or the sampled current data stream. The processing circuit is operably connected to the first and second converters, and receives information indicative of the sampled voltage data stream and sampled current data stream subject to any delay inserted by the phase correction circuit. The processing circuit then generates power consumption data from the sampled voltage data and sampled current data.
According to one embodiment of the present invention, the first and second converters are sigma delta converters. In such an embodiment, the phase correction circuit inserts the delay into the sampled data stream of one of the sigma delta converters. Accordingly, the power factor compensation technique described above allows the meter to benefit from the advantages of sigma delta conversion technology. In addition, the present invention eliminates the need for separately calibrated meters by providing a meter that optionally self-adjusts its calibration if the line frequency in which it is installed differs from the line frequency for which it was calibrated. For example, if a meter calibrated on a 60 Hz line is installed into a location that has a 50 Hz line frequency, the meter alters its calibration such that it is properly allocated for a 50 Hz line.
The above described features and advantages, as well as others, will become more readily apparent to those of ordinary skill in the art by reference to the following detailed description and accompanying drawings.